Our objective was to test the hypothesis that diurnal changes occur in thickness or volume of the femoral articular cartilage of the knee in asymptomatic young adults. Fat-suppressed three-dimensional (3D) spoiled gradient-echo magnetic resonance imaging (MRI) was employed. Six volunteers each were scanned early in the morning and at the end of a working day spent mainly standing. This protocol was repeated on 3 successive weeks. Femoral cartilage volumes were obtained via semiautomatic segmentation that employed a seeding algorithm. These segmentations then were regridded onto a 500-pixel template, and differences in the resulting thickness maps were assessed. Analysis of variance showed no significant diurnal variation in overall volume or thickness. The reproducibility for volume (test-retest coefficient of variation) was 1.6%. There were, however, statistically-significant diurnal changes in the thickness maps. Cartilage thickness decreased by up to 0.6 mm during the day in each of the following three specific locations: the patellofemoral compartment, the lateral tibiofemoral compartment, and the medial tibiofemoral compartment. Elsewhere, cartilage thickness was unchanged or increased by up to 0.5 mm. We conclude that, in asymptomatic young adults, cartilage volume does not change during the day; however, the cartilage does become thinner in locations that encounter the greatest biomechanical force.